1. Field of the Invention
The present invention relates to a method for producing a nitride-base ceramics, more specifically, a silicon nitride or oxynitride. The present invention particularly concerns heat treatment of a preceramic polymer mainly composed of polysilazane, polysiloxazane or polycarbosilazane, a precursor of the silicon nitride or oxynitride, to reduce their carbon content.
2. Description of the Related Art
Silicon nitride and silicon oxynitride have been attracting attention in various industrial fields due to their excellent properties, including mechanical strength, thermal shock resistance and electrical insulation. For making the shaped body with a high mechanical strength, compacted powders are sintered at near 2000.degree. C. generally, because of their poor shaping properties. To avoid this shaping difficulty, attempts have been made to shape a preceramic polymer or a precursor for silicon nitride or oxynitride and then pyrolyze it to obtain a silicon nitride or oxynitride retaining the original shape. Examples of such attempts are disclosed in the following publications:
Japanese Unexamined Patent Publication (Kokai) No. 49-20206 (Wolfgang Verbeek)
A shape made of a uniform mixture of silicon carbide and silicon nitride is produced by thermally decomposing a silazane at a temperature of about 200.degree. to 800.degree. C. to form a fusible carbosilazane resin. The carbosilazane resin is shaped into a required form, which is then heated at a temperature of about 800.degree. to 2000.degree. C. in an inert atmosphere.
Japanese Unexamined Patent Publication (Kokai) No. 49-69717 (Gerdhart Winter)
A shape comprised of a uniform mixture of silicon carbide and silicon nitride is produced by reacting at least one of halogenosilanes and ammonia to form a silazane. A composition comprising the silazane is shaped and then heated at a temperatur of about 800.degree. to 2000.degree. C. in an inert atmosphere, thereby decomposing the silazane to silicon carbide and silicon nitride.
J. Applied Polymer Sci., Vol. 27, 3751-3761 (1982) (B. G. Penn et al.)
A silicon carbon-silicon nitride (Si.sub.x N.sub.y C.sub.z) fiber is produced by thermally polymerizing tris(N-methylamine)methylsilane to obtain a polycarbosilazane resin and melt spinning the resin in an inert atmosphere. The resultant polycarbosilazane fiber is treated at 100.degree. C. in a wet atmosphere to make the fiber infusible, and then subjected to a heat treatment at 1500.degree. C. in a nitrogen atmosphere.
Japanese Unexamined Patent Publication (Kokai) No. 60-226890 (D. Seyferth)
A ceramic comprising a mixture of silicon carbide and silicon nitride is produced by shaping a polysilazane having a repeating unit of ##STR1## and comprising a plurality of precursor residues bonded to each other by an Si.sub.2 N.sub.2 bridge containing ##STR2## The shaped polysilazane is then heated at a temperature of up to 1420.degree. C. under a nitrogen flow, and the polysilazane is then decomposed into silicon carbide and silicon nitride.
The ceramics produced by the above processes are the mixture of silicon carbide, silicon nitride and, in most cases, free carbon, thus resulting in reduced electrical and thermal insulation properties due to the silicon carbide and free carbon content. Further, the silicon carbide and free carbon have a higher reactivity with aluminum than silicon nitride and, therefore, a composite of aluminum with ceramics comprising silicon carbide and free carbon as a reinforcing member may be deteriorated.